Experimental kinetic study of organic matter maturation: Time and pressure effects on vitrinite reflectance at 400 °C

At 400 °C, our experimental results demonstrate pressure and heating time to be important variables that promote VR increase and therefore the maturation of Type III organic material. VR increases with time at each investigated pressure. Despite rapid initial kinetics, the increase in VR decelerates with time at each pressure. When VR < 1.44%, increasing pressure reduces the rate of VR increase and hence retards the initial VR enhancement with time. The retarding effect of pressure on VR increase diminishes with enhancing VR. The retardation of VR increase is insignificant for geological maturation at 400 °C because a VR of 1.44% is attained in only a few hours. When VR > 1.44%, increasing pressure counteracts the deceleration of VR increase with time and thus greatly enhances the increase in VR with time. Evidently, vitrinite maturation takes place rapidly in a dry confined system and does not require addition of water to occur. The strong effect of the experimental heat-up on VR is obvious even for very short experiments and must be corrected in kinetic analysis. The evolution of VR with heating time (t) and pressure (P) at 400 °C from an initial VR of 0% is well described by our new power law rate equation VR(P,400°C,t)=(k(P,400°C)t)n(P,400°C), where the exponent n(P, 400 °C) and the rate constant k(P, 400 °C) increase with pressure. We regard this kinetic formulation as a step toward a general equation describing VR evolution as a function of time, pressure and temperature for Type III organic matter. The potential of the power law formalism to model VR from any starting VR and for complex metamorphic and heating time histories is shown by making explicit directions on how to use such a kinetic equation.